It is known that mechanical alloying is a solid-state process which uses the continuous fracturing and cold welding of powder particles to obtain the intimate aggregation of their atoms.
Such process consists in grinding material inside specific grinding mills: ball mills, planetary mills, attritor mills, gravity mills, vibration mills, which usually use balls as grinding means.
Once ground, the powdery material can be compacted in specific moulds and sintered to obtain the finished product.
Irrespective of the method and of the type of mill, during the grinding process a continuous and repetitive operation occurs involving the welding and fracturing of the powder particles which over time determines the total uniformity up to atomic level of the treated material.
In the case of alloy elements being introduced inside the mill which are different from one another, an alloy can be obtained in powder state in which the atoms of the different materials are intimately bonded to each other.
We nevertheless continue to speak of “mechanical alloying” even in the case of only one material being introduced inside the mill, in which case, the mechanical alloying process is aimed at grinding the material and refining its grains.
Mechanical alloying is called “complete” if the final alloy is completely uniform, while it is called “partial” if chemical-physical differences can be identified in different areas of the same powder particle or between different particles: in this latter case, anyway, an intimate contact is obtained between the different components of the mix and usually a microstructure with refined grain. The mechanical alloying process for the production of powders permits obtaining alloys and microstructures otherwise impossible to achieve by means of normal melting methods inasmuch as the process always occurs in solid state and, therefore, alloys or compounds can be produced far from the condition of thermodynamic stability.
This process also permits obtaining supersaturated alloys or metal alloys immiscible the one with the other, and simplifying the production of alloys in the case of the elements having very different melting points.
A classic example consists of the aluminium-tungsten (Al—W) binary alloy: tungsten has a melting point above the boiling point of aluminium and consequently such alloy cannot be produced using traditional melting techniques despite such components being soluble in one another in both liquid and solid phases.
During the mechanical alloying process, it is crucial to control the balance between the welding and the fracturing of powder particles.
If the powder particles bind together too much, then they tend to form blocks or agglomerates that prevent process continuity and the chemical-physical uniformity of the powder.
If, instead, powder fracturing is preponderant with respect to welding, the risk is that of obtaining a powder which is too fine to the extent of becoming pyrophoric in the metallic case, or in any case with an apparent density too low for the classic use in the powder metallurgy sector.
Welding and fracturing depend on the type of mill and on all the mechanical alloying process parameters: type of loaded powder, quantity, weight ratio between milling bodies and powder, grinding temperature, grinding time, grinding energy, etc.
Cold welding, furthermore, can occur not only between the powder particles but also between the powder and the surfaces of the mill and of the balls; such phenomenon very much limits the use of the mechanical alloying process, because it makes it inefficient from an energy viewpoint, difficult to control and with low yield in terms of ratio between the powder introduced into the mill and the powder removed from it.
To control the balance between welding and fracturing, use is commonly made of special control agents, or “PCA substances” (“process control agent”) or, more simply, “PCA”.
The PCA substances are usually organic compounds, oils, alcohol, organic acids, graphite or water, which regulate or limit the cold welding phenomena.
Part of the elements in the PCA combine with the metallic powder to form dispersoids, carbides or oxides, and part of them have to be removed from the powder before the consolidation and sintering phases, penalty the formation of blistering (i.e., the formation of bubbles inside the material due to the expansion of a gas) and low end mechanical properties.
Take for example the mechanical alloying of an aluminium-based alloy: by simply introducing the aluminium powder and relative alloy elements inside the grinding mill, only after a short time a complete cold welding of the aluminium on the mill walls and on the grinding balls is inconveniently obtained.
A PCA substance therefore has to be used which in this case is usually stearic acid in the quantity of 1-2% in weight with respect to the total weight of the material being worked.
The use of stearic acid as PCA regulates the cold welding phenomena and permits mechanical alloying: at the end of the grinding process, part of the PCA substance is still present and a degassing operation has to be performed to remove the residues so as to achieve a good final microstructure and sintering.
The use of stearic acid results in any case in obtaining a final alloy with a far from negligible content of carbon and oxygen in the form of carbides and oxides as initial components of the PCA substance; the final alloy is therefore chemically “polluted”.
Mechanical alloying is particularly problematic for all the alloys of the elements selected from the groups IV (titanium, zirconium and hafnium), V (vanadium, niobium, tantalum) and VI (chromium, molybdenum and tungsten) of the periodical table of elements, in particular those of group IV which have such a high reactivity that any commonly used PCA ends up introducing interstitial elements that are harmful for the obtained alloy.
In other words, the high chemical reactivity and the very high melting temperatures (from 1668° C. for titanium up to 3422° C. for tungsten) result in the alloys of these metals being produced with considerable difficulty by means of costly traditional manufacturing processes and methods.
For the alloys of the elements of group IV, e.g., starting with the relative purified oxide, the Kroll process or the Hunter process are used to obtain metal sponges of titanium, zirconium or hafnium; such sponges are the raw material for the subsequent melting processes required to eliminate the residues of chlorine, magnesium and sodium and to insert the alloy elements.
For the alloys of the elements of groups V and VI, instead, use is made of diverse thermochemical reactions which include aluminium-thermal reactions, reduction of oxides by means of hydrogen, reduction of oxides by means of carbon, use of potassium bi fluoride intermediates, etc., so as to obtain metallic powders subsequently sintered to obtain the final alloy.
The difficulty in producing these materials is increased by the fact that, to obtain high mechanical properties, especially tenacity, all the alloys of the above elements require a low level of interstitial elements, especially atoms of carbon, nitrogen, oxygen and sulphur which, if present, must be inconveniently removed.
As previously stated, the very need to obtain very low quantities of interstitial elements considerably prevents the adoption of the mechanical alloying method, in particular for alloys of titanium, zirconium, hafnium, vanadium, niobium and tantalum.
Such alloys are also very much affected by the cold welding phenomenon, which reduces the production output of the process and very much restricts the use of the mechanical alloying method for these materials, in some cases making it totally impossible if performed with common PCA.
In this respect, the patent document GB 2266097 proposes to use a certain quantity of tin as PCA substance for the mechanical alloying of titanium alloys.